Head-mounted display devices (also called herein head-mounted displays) are gaining popularity as means for providing visual information to a user. For example, the head-mounted display devices are used for virtual reality and augmented reality operations.
When using head-mounted display devices, it can be desirable to block out external or ambient light from entering a user's eyes in order to provide a fully immersive experience. However, in some circumstances, it may be desirable for the user to be able to observe the outside environment while using the head-mounted display device. For instance, a user may want to look down at his or her hands in order to view a keyboard, mouse, or controller, or to do things like grabbing a cup of tea, while using the head-mounted display device without having to remove the head-mounted display device.